As one of seat frames of a vehicle seat, there is known a seat frame which includes an outer frame member being a member with a hollow space such as a side frame forming the seat frame and is reinforced by filling the hollow space with foamed resin. In a seat frame disclosed in JP-T-2010-533616, a side frame of a back frame is formed in a member having the hollow space, and the hollow space is filled with foamed resin to reinforce the structure.
In the structure described above, since the hollow space is filled with the foamed resin, when a bending force is applied to the side frame, the cross-sectional shape hardly collapses, so that a bending strength and/or rigidity of the side frame can be improved compared to a case where the hollow space is not filled with the foamed resin. However, there is a problem that a weight of the seat frame is increased by being filled with the foamed resin.